Systematic optimization of a pyromellitic acid background electrolyte for capillary electrophoresis with indirect UV-vis detection and online pre-concentration analysis of thiosalt anions in treated mine tailings

Abstract

A CZE method using indirect UV-vis detection was developed for the simultaneous separation and determination of five thiosalt anions: sulfate (SO₄²⁻), thiosulfate (S₂O₃²⁻), trithionate (S₃O₆²⁻), tetrathionate (S₄O₆²⁻) and pentathionate (S₅O₆²⁻). As with many inorganic anions, absorbance by some thiosalt species in the UV-vis range is poor or non-existent. Fully ionized pyromellitate ion (PMA), with a high molar absorptivity, was evaluated as the chromophoric probe for analysis of these species. The factors considered in optimization of the method were: composition of the background electrolyte (BGE) (concentrations of PMA and electroosmotic flow (EOF) modifiers, pH of BGE, applied electric potential (V) and plug size of water for online sample pre-concentration. The optimal conditions were: [PMA] = 2.00 mM, [hexamethonium (HM²⁺)] = 0.80 mM, pH = 8.0, E = −30 kV and online sample pre-concentration (stacking) with 90 mbar s injection of water followed by 250 mbar s injection of thiosalt standard sample before application of separation voltage. The five thiosalt anions were separated in under 3 min with good resolution and sensitivity obtained for all the analyte peaks. Limits of detection (LOD) were between 0.09 and 0.34 μg mL⁻¹ without stacking; a three-fold increase in LODs to between 0.02 and 0.12 μg mL⁻¹ was achieved after online pre-concentration.